1. Field of the Invention
The present invention relates to an indicating instrument for a vehicle.
2. Description of Related Art
Conventionally, an indicating instrument for a vehicle that rotates a pointer in synchronization with a step motor is known. The pointer indicates a vehicle state value, which is displayed with a zero value as its reference, in accordance with a rotational position of the pointer. The indicating instrument for a vehicle is described in Japanese Patent No. 3770095 (corresponding to U.S. Pat. No. 6,624,608B2), for example. This indicating instrument rotates a pointer through the application of a drive signal of an alternating current, which alternates in accordance with an electrical angle, to a field winding of a step motor.
In the indicating instrument for a vehicle according to Japanese Patent No. 3770095, the pointer that is rotated in a zero-reset direction, which is a direction to return the pointer back to a position of a zero value, is stopped at a stopper position by a stopper mechanism. While the pointer is rotating in the zero-reset direction, an induced voltage is generated in the field winding of the step motor. When the pointer stops, the induced voltage decreases. Accordingly, the induced voltage generated in the field winding is detected, and if the detected voltage is equal to or smaller than a set value, it is determined that the pointer has stopped at the stopper position, and an electrical angle that corresponds to this stopper position is updated. As a result, even if the step motor loses synchronization due to disturbance such as vibration prior to start up of the instrument, so that a rotational position of the pointer is misplaced, the rotation of the pointer is controlled based on the updated electrical angle.
In the indicating instrument for a vehicle of Japanese Patent No. 3770095, the induced voltage is detected for each electrical angle where a voltage of one of the drive signals applied to the field windings in the shape of a cosine function and in the shape of a sine function is zero, i.e., at a zero point and electrical angles whose phases are shifted from the zero point by every 90 degrees. For this reason, a certain error may be observed in the electrical angle updated based on the detected voltage of the induced voltage, in a phase range within less than 90 degrees in a direction corresponding to the zero-reset direction, relative to an electrical angle that corresponds to an actual stopper position.
Furthermore, in the indicating instrument for a vehicle of Japanese Patent No. 3770095, even though the pointer has rotated to the stopper position, a speed of the pointer does not immediately decrease due to such as its elastic deformation. Therefore, the detected voltage of the induced voltage does not become equal to or smaller than the set value, so that the electrical angle may be advanced to the next detecting point whose phase is further shifted by 90 degrees. In such an abnormal state, the step motor loses synchronization when the electrical angle advances to a point whose phase is shifted by 180 degrees from the electrical angle that corresponds to the actual stopper position because of the above-described error of the updated electrical angle or vibration at the stopper position, for example, or when the phase is shifted by 180 degrees from the electrical angle that corresponds to the actual stopper position as a result of a flip-up of the step motor from a stopper. Due to this loss of synchronization of the step motor, the pointer rotates to a position that corresponds to an electrical angle whose phase is shifted by 360 degrees from a actual electrical angle. Accordingly, it is difficult to accurately control the rotation of the pointer after the loss of synchronization by the drive signal that corresponds to the electrical angle in the indicating instrument for a vehicle of Japanese Patent No. 3770095.